The present invention relates to the use of Code Division Multiple Access (CDMA) communication techniques in radio communication systems, and more particularly to systems and methods for selecting the number of carriers for a Direct Sequence Spread Spectrum Multiple Carrier (DS-SS MC) CDMA communication signal using a characteristic of a selected communication channel.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs, to maintain high quality service, and to avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next-generation technology. Furthermore, it is widely believed that Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to access networks to transmit voice, data, and/or video from the home, office, street, car, etc. will be provided by cellular service providers using a digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity.
Wireless communication systems transmit communication signals on one or more carrier waves. As used herein, the term xe2x80x9csignalxe2x80x9d refers to an electrical wave, either analog or digital, that is used to convey information, and the term xe2x80x9ccommunication signalxe2x80x9d refers to a signal that conveys user information such as, for example, voice, video, or data information. As used herein, the term xe2x80x9ccarrierxe2x80x9d is used to refer to a radio frequency (RF) wave generated at a transmitting station for the purpose of carrying a signal, which may be a communication signal.
In wireless communication systems, the term xe2x80x9cchannelxe2x80x9d refers to an electromagnetic communication path between a transmitter and one or more receivers. In many existing radio communication systems, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a channel is a single radio frequency band within a given frequency spectrum into which a communication signal""s transmission power is concentrated. Signals that can interfere with such a communication channel include those transmitted on adjacent channels (adjacent channel interference) and those transmitted on the same channel (co-channel interference). Interference from adjacent channels is limited by the use of band-pass filters that filter out energy outside the specified frequency band.
In TDMA systems, a channel comprises, for example, a time slot in a periodic train of time slots of a carrier having a given frequency. These time slots may be organized into groups called frames. A given user""s signal energy is confined to one or more of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the available time slots as well as by limitations imposed by channel reuse as described above with respect to FDMA.
With FDMA and TDMA systems (as well as hybrid FDMA/TDMA systems), one goal of system designers is to ensure that two potentially interfering signals do not occupy the same time and frequency. In contrast, Code Division Multiple Access (CDMA) allows communication signals to overlap in both time and frequency, while communication channels are defined by an encoding scheme, as discussed below. CDMA is a type of spread spectrum communication that has been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications since spread spectrum communication can be more robust against interference, allowing more signals to occupy the same bandwidth at the same time. Examples of commercial applications include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a CDMA system, an electrical signal embodying an informational data stream (e.g., digitized voice, data, video) to be transmitted is combined with an electrical signal embodying a higher bit rate data stream known as a signature sequence, or spreading sequence, to produce a spread spectrum signal. Each bit of the signature sequence is referred to as a xe2x80x9cchipxe2x80x9d, and the frequency of the electrical signal embodying the signature sequence is referred to as the xe2x80x9cchip ratexe2x80x9d. The ratio of the chip rate to the frequency of the electrical signal embodying the informational data stream is generally referred to in the art as the xe2x80x9cspreading ratioxe2x80x9d.
In an exemplary CDMA system, a spread spectrum signal may be generated by multiplying an electrical signal embodying an informational data stream and an electrical signal embodying a unique signature sequence. The information required to decode the spread spectrum signal (e.g., the unique signature sequence) may be transmitted to an intended receiver over a separate communication channel (e.g., a pilot channel or a control channel). Using this information, the intended receiver can extract the informational data stream from the spread spectrum signal, thereby establishing a communication channel with the transmitter.
In a wireless CDMA system, a plurality of spread spectrum signals may be combined at a transmitter to form a composite signal which modulates a radio frequency carrier, for example by binary phase shift keying (BPSK). In the composite signal, each of the spread spectrum signals overlaps all of the other spread spectrum signals in the time domain and the frequency domain. At an intended receiver, the composite signal is correlated with a signature sequence uniquely identifying one of the electrical signals embodying the informational data stream, such that the electrical signal embodying the desired informational data stream can be isolated and despread.
Traditionally, a signature sequence is used to spread one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a+1 or xe2x88x921, sometimes denoted xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. The signature sequence usually comprises G chips per information bit. The signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate two carriers at the same frequency, but ninety degrees different in phase. The entire G-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, e.g., a rake receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. If BPSK modulation is used, only the real part of the correlation value may be computed. When a large positive correlation results, a xe2x80x9c0xe2x80x9d is detected; when a large negative correlation results, a xe2x80x9c1xe2x80x9d is detected.
The xe2x80x9cinformation bitsxe2x80x9d referred to above can also be coded bits, where the code used is one or more of a block or convolutional code or an orthogonal code. Also, the signature sequence can be much longer than a single transmitted symbol, in which case a subsequence of the signature sequence may be used to spread the information bit. In many radio communication systems, the received signal includes two components: an I (in-phase) component and a Q (quadrature phase) component. This occurs because the transmitted signal has two components (e.g., quadrature phase shift keying, QPSK), and/or the intervening channel or lack of coherent carrier reference causes the transmitted signal to be divided into I and Q components. In a typical receiver using digital signal processing, the received I and Q component signals are sampled and stored at least every Tc seconds, where Tc is the duration of a chip.
In a multipath environment, a transmitted signal (e.g., a composite signal) follows several propagation paths from a transmitter to a receiver, typically as a result of the signal reflecting from one or more objects such as, for example, buildings, before arriving at the receiver. Since the several propagation paths are of unequal lengths, several copies of the transmitted signal may arrive at the receiver with different phases and time delays. The time lapse between the receipt of the first copy of the received signal and the final copy of the received signal is referred to as the delay spread of a channel. The number of resolvable paths for a particular carrier is a function of the delay spread of the channel and the chip duration for the spreading sequence. Hence, the number of resolvable paths for a particular carrier is proportional to the bandwidth of the transmitted signal.
A rake receiver provides a form of diversity combining by collecting the signal energy from the various received signal paths. Multipath diversity derives from the redundant communication paths in that when some paths fade, communication is still possible over non-fading paths. Thus, to provide multipath diversity, it is generally desirable for a modulated carrier to have sufficient bandwidth to support multiple paths. However, if a carrier supports too many paths, interference between copies of the transmitted signal traveling different propagation paths may result in degradation in performance of the rake receiver which may offset the gain obtained from diversity combining. This is particularly true in a multi-user environment due to the increase in interference levels.
Many existing cellular CDMA implementations utilize a single carrier to transmit the encoded information sequences. As discussed above, single carrier DS-SS CDMA systems commonly use orthogonal spreading codes in at least the forward link, i.e., the channel from a base station to a remote terminal. Each user is assigned one code from the set of orthogonal spreading codes. Assuming the channels are not affected by multipath fading, or are flat fading channels, signals from all CDMA users will remain orthogonal. Hence, the signal is not degraded by self interference or multiple access interference from other CDMA users. However, in channels subject to multipath fading, the orthogonality of the spreading codes may be lost because the reflected signals may lose their orthogonality. If the bandwidth of the carrier is large, such that the carrier supports a large number of paths, this may cause a single carrier DS-SS system to suffer from high self interference and multiple access interference.
Multiple carrier (MC) DS-SS CDMA systems segment the available frequency spectrum into a number of narrower-bandwidth modulated carriers. Thus, each carrier is subject to less frequency selective fading and supports fewer resolvable paths. This reduces both the self interference and the multiple access interference for communication channels in the forward link of a CDMA system. Multipath diversity, which is normally provided by using a rake receiver, may be replaced by frequency diversity. Also, a sufficient degree of multipath diversity is obtained by relatively few paths. DS-SS MC CDMA systems may utilize multiple carriers in the forward link while utilizing a single carrier in the reverse link.
An existing architecture for DS-SS MC CDMA signal design is illustrated in FIG. 1. In general, and referring to FIG. 1, an electrical signal embodying an informational data stream to be transmitted, which may comprise a serial stream of bits, each of which is T seconds long, is de-multiplexed into N parallel branches using de-multiplexer 20. Each data bit is then spread by a respective spreading sequence that has G chips and is Nxc3x97T seconds long. Each spread data bit modulates S respective carriers in modulators 21 according to a modulation scheme such as BPSK, QPSK or another convenient scheme. All Sxc3x97N=M carriers are then combined by a suitable device such as, for example, summer 22 and transmitted as a composite signal. Other CDMA techniques described in the literature can be considered a special case of the above, where N or S may take the value 1.
As discussed above, in cellular DS-SS MC CDMA communication systems users are allocated different channels (e.g. spreading codes). Because different users may be located at different geographic positions with respect to a base station, the different channels may be characterized by different delay spreads Td. Additionally, because individual users may move relative to a base station during the course of a call, a user""s connection may have different delay spreads at different points in time during the call. Hence, various users may require a different number of carriers, M, such that each carrier provides a desired number of paths, L, to support multipath diversity. At the same time, the number of paths per carrier, L, should not be so large that self-interference and multi-access interference are too large. Known DS-SS MC CDMA signal design techniques essentially perform a pre-decided assignment of a fixed number of carriers of the MC CDMA signal regardless of the channel conditions. Thus, existing signal design techniques do not provide the ability to select a desired number of carriers to allocate to a particular signal based upon a characteristic of the communication channel.
Accordingly, there is a need in the art for improved systems and methods for designing and configuring DS-SS MC CDMA signals.
The present invention addresses these and other problems by providing systems and methods for selectively allocating a number of carriers to transmit a DS-SS MC CDMA signal based upon a characteristic of a radio communication channel assigned to carry the signal. The present invention uses new signal design techniques to allocate a desired number of carriers for a signal based upon characteristics of the channel over which the signal is to be transmitted. In one embodiment, the invention selectively allocates a number of carriers to a signal based upon the delay spread associated with the selected channel. The number of carriers allocated to a signal may be selected to improve a performance parameter of the network. For example, given a fixed total bandwidth, the number of carriers allocated to a signal, and hence, the bandwidth of each carrier, may be selected to provide a desired number of paths on each carrier transmitting the signal. In another embodiment, the invention selectively allocates a number of carriers to a signal based upon a desired class of service for an information stream to be transmitted on the channel. Preferably, when the signal carries a multitude of information streams, such as multimedia systems, information streams that require a higher class of service are allocated a greater number of carriers.
In one aspect, the invention provides a method of processing a communication signal. The method comprises the steps of determining a desired number of paths for a selected channel, and allocating at least one carrier to the communication signal that corresponds to the desired number of paths. A desired number of paths may be determined by referencing a memory location associated with the communication system that contains information indicative of a desired number of paths for the selected channel. Using an estimate of the delay spread associated with the selected channel, the number of carriers can be decided.
In another aspect, the invention provides a method of generating a communication signal. The method comprises estimating a selected channel""s delay spread, determining a desired number of paths per carrier for the selected channel based, at least in part, on the selected channel""s delay spread, and allocating at least one carrier to the signal that corresponds to the desired number of paths.
In another aspect, the invention provides a method of generating a communication signal. The method comprises determining a characteristic of a selected channel, and, based upon the characteristic of the selected channel, allocating a number of carriers to the communication signal.
In another aspect, the invention provides a system for processing a communication signal in a communications system in which user communications are assigned to a selected active channel. The system comprises a determining circuit for generating a first signal representative of a desired number of paths for said selected active channel of said communications system, and an allocating circuit for generating a second signal representative of at least one carrier allocated to said communication signal in response to said first signal.